Along with the pursuit of smaller sizes, increased functions, and faster communications in electronic equipment, further higher densities of the circuit boards which are used for the electronic equipment have been sought. To meet such demands for higher densities, circuit boards are being made multilayered. Such multilayer circuit boards are, for example, formed by taking an inside layer board which is comprised of an electrical insulating layer and a conductor layer which is formed on its surface, laminating an electrical insulating layer over it, forming a conductor layer over this electrical insulating layer, and further repeating this lamination of an electrical insulating layer and formation of a conductor layer.
As the material for forming the electrical insulating layer of such multilayer circuit boards, in general ceramics and thermosetting resins are being used. Among these, as thermosetting resins, epoxy resins are being widely used since they are excellent in the point of the balance of economy and performance.
As an epoxy resin material for forming an electrical insulating layer, for example, Patent Document 1 discloses an epoxy resin composition which contains (A) an epoxy resin, (B) an active ester compound, and (C) a triazine-containing cresol novolac resin. According to this composition, it is possible to form an insulating layer which exhibits a high bonding force to the plating conductor, regardless of the roughness being small, and is low in linear expansion rate and low in dielectric tangent.
In this regard, in the above multilayer circuit board, the conductor layers are connected with each other through via holes which are provided in the electrical insulating layers. In the formation of via holes, an electrical insulating layer is formed with a hole for via hole by laser processing, then metal plating, but before that, desmearing is performed to remove the resin residue (smears) which formed in the laser processing and remains on the lower conductor layer or electrical insulating layer. The desmearing is performed by, for example, dipping the multilayer board in which the hole for via hole are formed in a solution of a chemical oxidizing agent such as potassium permanganate or potassium dichromate to dissolve away the smears in the hole. If the desmearing ability is insufficient and the desmearing is not sufficiently secured, even if metal plating the via hole, the smears are liable to prevent conduction between the upper conductor layer and lower conductor layer from being sufficiently secured.
Further, in the multilayer circuit board, the conductor layer and the electrical insulating layer have to be closely bonded, but if the adhesion is weak, peeling may end up occurring during the process of fabrication or during the mounting of the multilayer circuit board and further during use as a board for an electronic material and reliability is liable to no longer be insufficiently secured.